DFMS mass spectra in the m/z ranges of 36 and 38. The spectra demonstrate the clear identification of the two isotopes 36Ar and 38Ar and of the interfering molecules. The exact m/z locations are given in the text. The spacecraft background spectra were obtained before the cometary signal became apparent (2 August 2014, heliocentric distance of 3.6 AU, almost 800 km from the nucleus).
This measurement adds to the debate about the role of comets in delivering various “ingredients,” such as water, to Earth. Comets are considered to be representative of icy planetesimals that may have contributed a significant fraction of the volatiles to planets in the very early solar system. It is also believed that comets must have brought some water to the Earth, however, the magnitude of their contribution is still a matter of debate. Hans Balsiger from the Physics Institute at the University of Bern and his team were now able to detect the noble gas argon in the coma of comet 67P/Churyumov-Gerasimenko. It is for the first time that researchers were actually able to measure this particular gas on a comet.
Although the abundance-range of argon to water was quite large, it could be used to investigate the question of whether comets brought water to Earth, Balsiger explains: “The argon-to-water ratio on Earth is several magnitudes lower than observed at 67P/C-G. If it were the same here on Earth as on the comet, we’d have far greater quantities of argon in our atmosphere.” The results of the study which is part of ESA’s Rosetta comet mission were published in the journal Science Advances.
D/H versus 36Ar/H2O mixing of 67P/CG-like and asteroidal materials. The asteroidal composition is represented by the Orgueil (CI) and Murchison (CM) carbonaceous chondrites. CI/CM chondrites are considered as the best representatives of volatile-rich primitive meteorites (19). Cometary data: this work, Altwegg et al. (10). Meteorite data: Mazor et al., Bogard et al., and Kerridge (20–22). Earth data, surface inventory: Lécuyer et al. and Ozima et al. (12, 13). Range of estimates for bulk Earth: Marty and Halliday (17, 18).
One of the main goals of the Rosetta mission is to measure in situ the volatile inventory of 67P, a so-called Jupiter family comet. The tool used to perform these measurements is called ROSINA, a mass spectrometer built at the University of Bern. It has a high sensitivity and a high mass resolution, which allows the comparison to remote sensing observations and to the only previous in situ measurement at comet Halley in 1986. According to Balsiger, information such as this provided by ROSINA could not be obtained from meteorites collected on Earth (which were the only source of deep-space samples available for investigations until now).
In October 2014 ROSINA detected 2 isotopes of argon during a fly by at a distance of only 10 kilometers from “Chury’s” surface. During a period of four days researchers compared the abundance of argon with that of other molecules in the coma. “The outcome of this estimate is that contributions from comets such as 67P to the Earth’s oceans are, if at all, very small,” Balsiger comments. Earlier data provided by ROSINA about the ratio of hydrogen and deuterium on the comet already pointed in that direction.
(A and B) Comparison of argon abundances to water (A) and to molecular nitrogen (B). (A and B) Relative abundances of 36Ar versus H2O (A) and N2 (B). 36Ar abundances were measured relative to water and molecular nitrogen during four periods in October 2014, when Rosetta was close to 67P/CG (10 km). Individual measurements cover 20 s. Measured particles per 20 s are plotted. Ratios are molecular ratios. (A) Large spread of the relative abundances due to the high temporal variability and spatial heterogeneity of the coma (7). (B) Good correlation between 36Ar and N2 due to their similar volatility.
In situ noble gas measurements are important complements to the measurements of deuterium in water and the nitrogen molecules in the coma of 67P. They furthermore address the question of formation temperature of the comet’s original material; icy grains that formed the planetesimals 4.5 billion years ago in the very cold regions of our solar system beyond the orbit of Neptune. http://www.alphagalileo.org/ViewItem.aspx?ItemId=156713&CultureCode=en
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